Apparatus for cracking hydrocarbon oils



Patented Mar. 13, 1934 UNITED STATES- APPARATUS FOR CRACKING HYDROCARBON OILS Enslo s. Dixon, Port Arthur, Tex, assignor'to The Texas Company, New York, N. Y., a corporation of Delaware No Drawing.

1'7 Claims.

This invention relates to apparatus for thermally decomposing hydrocarbon'oils to form gasoline and more particularly to improved tubular heating means adapted to resist the action of 5 ;orrosive substances contained in the oil and to withstand continued exposure to the temperatures employed in heating the oil which is undergoing treatment.

The invention contemplates improved tubular heating elements for use in a continuous coil type of heater adapted for heating and converting hydrocarbon oil which are formed from an alloy consisting predominantly of iron and chromium and in which the chromium content is maintained within certain definite limits sufiicient to impart substantially complete resistivity towards the action of the corrosive constituents of the oil without imparting embrittling tendencies which may develop during continued exposure oi the heating elements to' the relatively high temperatures ordinarily employed in cracking the oil.

Heretofore considerable difficulty has been experienced "in cracking operations particularly when cracking hydrocarbon oil containing appreciable amounts of sulfur due to the extremely rapid and dangerous rate of deterioration of those metal portions of the apparatus coming into immediate or direct contact with the oil undergoing heat treatment. This deterioration is the result of the corrosive action upon the metal of the sulfur containing bodies in the oil which for the most part are converted during heating of theoil into the form of hydrogen ulfide which, as is generally known, attacks ordinary steel extremely rapidly. These difficulties are enhanced when employing relatively high temperatures in the cracking operation since with increasing decomposition of the oil there may be an increasing and simultaneous increase in the decomposition of the sulfur compounds contained in the oil with correspondingly greater evolution of hydrogen sulfide, and also as a result of the relatively high temperatures being employcd these compounds become correspondingly more active towards the metal with which they are coming into contact. Consequently in cracking operations, particularly in the coil and drum type of operation, where portions of the coil may be maintained at relatively high temperatures for prolonged periods of time and in which the oil passing thru the coil undergoes a substantial amount of conversion, the rate of deterioration has been extremely great and the use of tubular heating coils formed from ordinary iron Application December 4, 1930, Serial No. 500,139

and steel has not only been costly but fraught with considerable danger. I

In orderto overcome these dimculties various methods have been employed in the prior art toprotect the exposed surfaces of the metal against the attack of the corrosive constituents of the oil, as for example coating or spraying of the surfaces with some sort of refractory material or else treating the surface of the metal by such methods as calorizing in order to attempt to impart desirable corrosion resistant qualities thereto, but none of these methods have been attended with any material degree of success.

Different metals and their alloys have also been used in the fabrication of cracking vessels and related apparatus, particularly steels containing relatively large amounts of chromium as well as certain other metallic constituents. The patent to Curme No. 1,646,349, dated October 18, 1927 describes an alloy steel containing 20% and more of chromium and which was proposed for the thermal decomposition of hydrocarbon oil more particularly from the standpoint of inhibiting deposition of carbon from the oil upon the metal surface during the decompo- $0 sition.

It has been found, however, that these high chromium steels and also those containing somewhat lower percentages ,of chromium such as the so-called stainless steels, containing from 12% to 14% of chromium, develop brittleness during extended exposure to the conditions ordinarily prevailing in furnaces adapted for heating and converting hydrocarbon oil at temperatures of around 900 F. and in which case the exterior portions of the heating coil may be subjected to temperatures of 1300 F. and higher. Tubes formed from steel containing around 19 or 20% of chromium after comparatively short exposure to such temperature conditions may frequently be fractured and broken merely by tapping with a hammer and this notwithstanding their much greater initial tensile strength as compared with steel commonly used for tubular heating coils.

It has been attempted in the prior art to overcome the development of this undesirable characteristic in high'chrome steel by incorporating in such steel various amounts of nickel, silicon or other substances as has been described in the patent to Norwood No. 1,703,949, dated March 5,

- Later developments have lead to the production of special alloys such, for example, as 18-8 metal consisting ofaround 18% chromium and about 8% nickel, the rest being predominantly 1 iron. In this latter alloy the components appear of tubes fabricated from this type of material without their having been exposed to unduly severe heating and without any thinning down or even any apparent elongation or creep of the metal prior to fracture. In fact the frequency of these unexpected failures has warranted the replacement of such tubes with tubes drawn from ordinary steel on account of the apparently greater, durability and reliability of the latter in this type of service even tho subject to materially greater deterioration by corrosion.

I have found after extended investigation with alloys containing various amounts of chromium that a tubular heating coil formed from an alloy consisting predominantly of iron and chromium, but in which the chromium is confined to' about 5%, is particularly well adapted to the heating and subsequent conversion of sulfur bearing hydrocarbon oils since such material has been found to be quite effective in resisting the corrosive action of the sulfur constituents in the oil and at the same time is capable of enduring extended or prolonged heating in fired furnaces, such as used in cracking processes, without the development of brittleness or change in its crystal structure which would otherwise result in its failure by bursting or splitting.

The proportions of the various constituents of steel which possesses the foregoing characteristics are preferably as follows:

Chromium 4.00% to 6.00%

Carbon 0.05% to 0.25% Manganese 0.30% to 0.60% Silicon 0.10% to'2.00%

, Extensive experiments have shown that tubular heating elements formed from a steel of the above composition and containing not appreciably less than 4% and not more than 7% of chromium are remarkably resistant towards corrosion, paricularly the type of corrosion peculiarto the cracking of sulfur bearing oils, especially considering the fact that as regards other properties, such as its resistance towards scaling or towards oxidation by the combustion. gases in furnaces, "t does not differ very much from ordinary carbon steel. In the past it has been the practice to incorporate appreciably larger amounts of chromium than this in steel alloys which were to possess desirable corrosion resisting qualities web as the so-called stainless steels, for example, which usually contain around 12% or more of I chromium.

The tubes of my invention, constructed from steel containing between 4% and 6% of chro- "alum, have been found to consistently possess a useful life of at least four times that of ordinary steel and may frequently average considerably higher than this. Aside from greatly increased life such tubes are, of course, capable of withstanding greater working pressures such weaves as may be encountered when cracking at superatmospheric pressures, since steel containing this amount of chromium possesses around 50 greater tensile strength than carbon steel at the temperatures employed in cracking.

The stainless steels of the prior art already referred to are difflcultly workable on account of their hardness which has been imparted thereto due to their relatively high chromium content.

It is characteristic of chrome steels in general to air harden after being heated up to suitable temperatures for working, or upon cooling down after service in cracking furnaces and this characteristic becomes more pronounced with increasing chromium content. chrome steel containing not over 7% chromium lies in the fact that its properties are not so materially difierent from those of ordinary carbon steel particularly with respect to its workability. Thus when forming tubes from billets of this low chrome steel the piercing temperature may be substantially the same as that prevailing when forming tubes from billets or ordinary carbon steel, whereas in the case of higher chromium content steels such as stainless steel the piercing temperatures are of necessity materially higher. Aso it is not subject to harden to any greater extent during working than does ordinary carbon steel and which is to 'be contrasted with the very pronounced work-hardening property of the special alloys containing chromium and nickel.

The crystal or particle structure of my low chromium content steel does not undergo any apparent change during exposure to the conditions of heating prevailing in the ordinary cracking furnace An advantage of my low dication of grain or crystal growth nor does any embrittlement develop during prolonged heating, as is apparently the case with relatively higher chrome steel.

Experience seems to indicate that the high chrome nickel alloy steels, which are austenitic, are unstable particularly at temperatures of around 1300 F., or when exposed to heating at temperatures in the range prevailing inythe tube bank of fm'nacesemployed for cracking hydrocarbon oil. It seems that during exposure to such temperatures such steel loses its austenitic nature, becomes magnetic and may be converted into a system of two or more phases possibly as a result of some such action as carbide precipitation at the grain boundaries wherein a certain amount of the chromium separates from the iron to associate itself more closely. with the carbon for which latter it possesses considerable aflinity, and it is known that this action may be accompanied by a weakening of the metal structure. In order to minimize this carbide precipitation it is the practice in manufacturing these alloys to reduce the carbon content as much as possible altho even with as low as 0.07% carbon the alloy is still subject to the above type of failure. I

I have found that there is apparently no pe'rceptible change in the internal structure of steel containing from 4 to 6% chromium even though it may contain more than three times as much carbon as that mentioned above and that it is 1,eeo,7ao

' ally subjected to overheating such portions of the tube may tend to bulge or stretch'in the same manner as a carbon steel tube, whereas in the case of a high chrome steel tube, particularly such as may be formed from 18--8 metal, where cracks appear without any apparent local overheating as is ordinarily evidenced by the appearance of a red spot, such cracks are unaccompanied by any bulging or any deformation of the adjacent portion of the tube. Moreover, the adjacent faces of the resulting fracture in the latter instance have a grainy and crumbly appearance.

The fact that such failure in the case of these chrome nickel steels occurs without plastic deformation of the metal indicates a progressive loss in desirable properties of elasticity under "-uch conditions, and indicates that the material fails as a rule within its initial elastic strength, and which is a further indication that embrittlement has taken place. Steel containing around 5% chromium on the other hand retains these desirable qualities which make it more durable as well as-render its use in cracking furnaces extremely safe, notwithstanding the fact that the above mentioned high chrome nickel steel due to its relatively greater alloy content may possess .0 or 40% greater initial tensile strength under cracking furnace temperature conditions.

The retention or the elastic properties as in the case of the low chromium content steel permits tubes formed from this material to undergo considerable swelling before failing, which condition may be brought about by unintentional localized overheating for example. In this way swelling of the tube provides sumcient warning so that furnace conditions may be remedied or other steps taken to prevent bursting of the tube, which in the case of oil cracking furnaces may result in disastrous and dangerous fires.

Due to its lower chromium content my low chrome steel can be manufactured. at a substantially lower cost than the high chrome alloys and on account of the facility with which it may be worked. the cost of tubes formed from such steel is not appreciably greater than that of carbon steel. Moreover, due to its low chromium content tubular members or connections formed from it may be ball jointed and otherwise worked or machined'with less difficulty than in the case of members or articles fabricated from higher chromium content steel such as the stainless steels.

The coefiicient of expansion of steel containing around 5% chromium is substantially the same as that of carbon steel as may be contrasted either with stainless steel whose coemcient of expansion 1'", appreciably less than that of ordinary steel or on the other hand with chrome nickel steels such as l88 whose coefficient of expansion is greater than that of ordinary steel. its coemcient of expansion is substantially the same as that of carbon steel. no difliculty is experienced in making leak-proof connections between tubes or pipes formed from low chrome steels and fittings or junction boxes fabricated from ordinary steel.

Consequently, since While I have discussed the formation of tubu- I lat heating elements from chromiumsteel in which the chromium ranges from 4 to 6% it is contemplated that the use of this alloy may of course also be extended to include the fabrication of pipe connections and various fittings such as may be necessary in assembling tube sections to form a'continuous tubular heating coil which may operatively communicate with reaction vessels and the like to form apparatus adapted for the cracking of hydrocarbon oils in either the liquid or the vapor phase.

Obviously many modifications and variations of the invention,.as hereinbefore set forth, may be made without departingfrom the spirit and scope thereof, and therefore, only such limitations should be imposed as are indicated in the appended claims.

I claim; j s

1. As a new article of manufacture, a tube adaptedfor the direct fire heating of. hydrocar- "bon oils to raise such oils to a high or cracking temperature composed of an alloy consisting predominantly of iron and, chromium and substantially free from nickel and in which the chromium content is approximately 4 to 6%.

2. As a new article of manufacture, a tube adapted for the direct fire heating of sulfur bearing hydrocarbon oils to raise such oils to a high or cracking temperature composed of an alloy consisting predominantly of iron and not appreciably less than 4% and not over 7% chromium and substantially free from nickel and in which the chromium content is suflicient to render the tube substantially resistant to the corrosive attack of the sulfurous constituents of the oil and is insufiicient to cause development of embrittlement of the metal during extended heating.

3. As a new article of manufacture, a tube adapted for the direct fire heating of hydrocarbon oils to raise such oils to a high or cracking temperature composed of an alloy consisting of 4 to 6% of chromium, .05 to .25% of carbon, .3 to 13% of manganese, .1 to 2% of silicon and the balance iron.

4. A process of treating hydrocarbon oils which comprises subjecting the oil to a high or cracking temperature in a container composed of an alloy consisting predominantly of iron and chromium and substantially free from nickel and in which the chromium content is approximately 4% to 6%.

5. A process of treating sulphur bearing hydrocarbon oils which comprises subjecting the oil to a high or a cracking temperature in a con- -tainer composed of an alloy consisting predominantly of iron and not appreciably less than 4% and not over 7% chromium and substantially free from nickel, and infwhich the chromium content is sufficient to render the container sub- 135 stantially resistant to the corrosive attack of'the sulfurous constituents of they oil and is insuflicient to cause development of embrittlement of the metal during extended heating.

6. A process of treating hydrocarbon oils 140 which comprises subjecting the oil to a high or a cracking temperature in a tube adapted for direct fire heating composed of an alloy consisting predominantly of iron and chromium and substantially free from nickel and in which the chromium content is approximately 4% to 6%.

7. A cracking coil for hydrocarbon oils comprising a plurality of cracking tubes, said tubes being composed of. an alley of chromium andiron and substantially free from. nickel and in which the chromium content is approximately 4% to 6%.

8. A cracking coil for hydrocarbon oils comprising a plurality of cracking tubes, said tubes being composed of an alloy of chromium and iron and substantially free from nickel and in which the chromium content is not appreciably less than 4% and does not exceed 7%.

9. A process of converting hydrocarbon oils which comprises subjecting the oil to a cracking temperature while in contact with a container composed of an alloy of chromium and iron and substantially free from nickel and in which the chromium content is not appreciably less than and does not exceed 7%.

10. A cracking coil for hydrocarbon oils comprising a plurality of interconnected tubes, said tubes being composed of an alloy of chromium and iron and in which the chromium content is approximately 4% to 6%.

11. A process of converting hydrocarbon oils which comprises subjecting the oil to a cracking temperature while in contact with a container composed of an alloy of chromium and iron and in which the chromium content is approximate-v ly 4% to 6% 12. An apparatus for use in a process wherein hydrocarbon fluids are subjected to thermal conversion which comprises a container adapted to receive the hydrocarbon fluid composed of an alloy of chromium and iron in which the chromium content is approximately 4% to 6%.

13. An apparatus for use in a process wherein hydrocarbon fluids are subjected to thermal conversion which comprises a container adapted mooos'eo to receive the hydrocarbon fluid composed of an alloy of chromium and iron and in which the chromium content is not appreciably less than 4% and does not exceed 7%.

' 14. An apparatus for use in a process wherein hydrocarbon fluids are subjected to thermal conversion which comprises a container adapted to receive the hydrocarbon fluid composed of an alloy of chromium and iron and substantially free from nickel, and in which the chromium content is approximately e% to 6%.

15. An apparatus for use in a process wherein hydrocarbon fluids are subjected to thermal conversion which comprises a container adapted to receive the hydrocarbon fluid composed of an alloy of chromium and iron and substantially free from nickel, and in which the chromium content is not appreciably less than 5% and does not exceed 7%.

16. A process of treating hydrocarbon fluids which comprises subjecting the hydrocarbon fluid to a temperature sufllciently high to effect thermal conversion thereof while in contact with a container composed of an alloy of chromium and iron and in which the chromium content is approximately 4% to 6%.

17. A process of treating hydrocarbon fluids which comprises subjecting the hydrocarbon fluid to a temperature sufllciently high to effect thermal conversion thereof while in contact with a container composed of an alloy of chromium and iron and in which the chromium content is not appreciably less than 4% and does not exceed 7%.

ENSLO S. DIXON. 

